Production of castings of magnesium and magnesium alloys in permanent molds



nent molds it is the Patented June 7, 1932 UNITED STATES PATIENT OFFICE ADOIIF BECK, 0F BITTERFELD, GERMANY fnonucrron or cnsrmes or MAGNESIUM AND MAGNESIUM ALLOYS m PERMANENT MOLIDS 1T0 Drawing. Application filed July 11, 1930, Serial No. 467,390 and in Germany August 10, 1929.

The present invention relates to an improved process for casting magnesium and magnesium alloys in permanent molds.

When casting metals into metallic permagenerahpractice to provide the interior surfaces of the molds with a black-wash which prevents direct contact of the molten metal with the mold surface. The black-wash present in the-mold allows of an easy separation of the finished casting from the mold and at the same time causes a smooth and perfect surface on the casting."

The black-washes hitherto generally employed in the production of similar castings of aluminium alloys have proved insufficient when casting magnesium and its high percentage alloys. On the other hand, attempts to utilize for this purpose the known protective substances generally required when aocasting magnesium alloys into said molds have also ended unsuccessfully. This result is probably due to the fact that practically all of these protective substances split off gases in contact with the molten metal, the

gases being entrapped by the solidifying metblack-wash which on heating develop, if any,

al and thus causing the formation of blowholes and similar defects in the castings.

A method which oifereditselftherefore was to employ such protective substances as a only imperceptible quantities of gas. When employing f. i. boracic acid for this purpose it became apparent that the adherence of that substance which in contact with the molten metal is caused to fuse on the surface of the mold, is somewhat irregular: in some cases, the boracic acid is swept along with the metal running into the mold; frequently also boracic acid adheres ished casting than to the surface of the mold so that when separating the casting from the mold also the black-Wash is removed whereas other sections of the mold rema n covered with the black-wash. Because of this irregular behaviour boracic acid alone is unsuitable for the production of permanent mold castings of magnesium and'magnesium alloys on a technical scale.

According to the present invention, boracic acid is employed in combination with submore rigidly to the fin-v stanr'es that cause form adherence of the boracic acid on the mold surface. Substances of this kind are various metal oxidessuch as magnesium oxide (MgO, preferably in the precipitated form), iron oxide (Fe O alumina (A1 0 further graphite, levigated chalk, mixtures of MgUl and MgO in proportions resembling those prevailing in the so-called sorel-cements i. e. setting mixtures of MgO with MgCl and water, preferably in approximate proportions of lMgCl z5MgO. The best results were obtained with F 0 in combination with boracic acid as the castings produced on the inner surface of the molds do not only rigidly adhere to the latter but also display an extremely fine structure resulting in a very smooth surface of the castings. A further advantage attending the application of these black-washes consists in that they promote thermal isolation of the molten metal from the mold proper, thereby perceptibly retarding solidification of the metal in the mold, which is frequently desirable.

Examples of suitable composition are the following (parts by weight) 1. 1 part of magnesia usta 1 part of powdered boracic acid,

2. 1 part of magnesia usta 1 part of powdered anhydrous magnesium chloride 1 part of powdered boracic acid,

3. 1 part of powdered graphite of powdered boracic acid,

4. 1 part of iron oxide 1 part of powdered boracic acid.

It will be understood that the proportions stated in the foregoing examples are by no means imperative as they may widely vary provided the proportion of boracic acid in the mixture amounts to at least about 25 percent of the total.

The black-washes according tothe present invention are advantageously applied by dusting the preferably preheated inner surface of the mold with a well sifted intimate mixture of one or several of the adhesive substances named with boracic acid. It is also possible, however, to spray a, preferably aqueous or alcoholical, "suspension of the black-washes on to the mold surface, the

: 1 part suspending medium being evaporated in contact with the preheated mold.

I claim:

1. In a process of casting magnesium and magnesium alloys into permanent molds the step which consists in applying-a black-wash comprising boracic acid and a substance promoting the adherence of boracic acid to the metallic inner surface of the mold.

2. In a process of casting magnesium an magnesium alloys into permanent molds the step which consists in applying a black-wash comprising an intimate mixture of boracic acid with a metal oxide to the metallic inner surface of the mold.

3. In a process of casting magnesium and magnesium alloys into permanent molds the step which consists in applying a black-wash comprising an intimate mixture of about equal parts of boracic acid and an oxide of the group of metals consisting of magnesium, aluminum, and iron to the metallic inner surface of the mold.

l. In a process of casting magnesium and magnesium alloys into permanent molds the step which consists in applying a black-wash comprising an intimate mixture of boracic acid and an oxide ofthe group of metals consisting of magnesium, aluminium and iron to the metallic inner surface of the mold, the proportion of boracic acid in the mixture being at least about 25 percent by weight of the total.

5. In a process of casting magnesium and magnesium alloys into permanent molds the step which consists in applying a black-wash comprising an intimate mixture of boracic acid and graphite to the metallic inner surface of the mold, the proportion of boracic acid in the mixturebeing at least about 25 percent by weight of the total.

6. in a. process of casting magnesium and magnesium alloys into permanent molds the step which consists in applying a black-wash comprising an intimate mixture of boracic acid and levigated chalk to the metallic inner surface of the mold, the proportion of boracic acid in the mixture being at least about 25 percent by weight of the total.

7. In a process of casting magnesium and magnesium alloys into permanent molds the step which consists in applying a black-wash comprising an intimate mixture of about equal parts by weight of boracic acid and iron oxide (Fe O to the metallic inner surface of the mold.

In testimony whereof I have hereunto set my hand.

ADOLF BECK. 

